1. Field of the Invention
The present invention generally relates to a power output apparatus and a method of controlling the same. More specifically, the invention pertains to a power output apparatus for transmitting or utilizing power output from an engine at a high efficiency and a method of controlling such a power output apparatus.
2. Description of the Related Art
In proposed power output apparatuses mounted on a vehicle, an output shaft of an engine is electromagnetically connected to a drive shaft linked with a rotor of a motor via an electromagnetic coupling, so that power of the engine is transmitted to the drive shaft (as disclosed in, for examples JAPANESE PATENT LAYING-OPEN GAZETTE No. 53-133814). When the revolving speed of the motors which starts driving the vehicle, reaches a predetermined levels the proposed power output apparatus supplies an exciting current to the electromagnetic coupling in order to crank the engine, and subsequently carries out fuel injection into the engine as well as spark ignitions thereby starting the engine and enabling the engine to supply power. When the vehicle speed is lowered and the revolving speed of the motor decreases to or below the predetermined levels on the other hand, the power output apparatus stops the supply of exciting current to the electromagnetic coupling as well as fuel injection into the engine and spark ignition, thereby terminating operation of the engine.
In the vehicle with such a power output apparatus incorporated therein, power required to drive auxiliary machinery is obtained through the rotation of the output shaft of the engine or compensated with the electric power stored in a battery. The auxiliary machinery includes auxiliary machines required for the operation of the power output apparatus, for example, a pump for circulating the cooling water of the engine, and those required not for the operation of the power output apparatus but for the operation of the vehicle, such as a pump for a power steering and a pump for an air conditioner. In the system of supplying the power of the auxiliary machinery from the battery, the battery is required to have a large capacity and thereby a large size to supply sufficient electric power to the auxiliary machinery. The battery is generally charged with the regenerative electric power corresponding to the slip of the electromagnetic coupling. The total energy efficiency is thus lowered depending upon the operation efficiency of the electromagnetic coupling and the charging efficiency and discharging efficiency of the battery.
In the system that the auxiliary machinery is mechanically linked with the output shaft of the engine, mechanical energy produced by the rotation of the output shaft is directly supplied as the power of the auxiliary machinery. This structure does not cause the problem as discussed above. In this structure, however, the output shaft of the engine does not rotate when the engine stops its operation and the vehicle is driven only with the electric power stored in the battery. The standing output shaft does not produce the power required for the operation of the auxiliary machinery.